lib/systems: uname.processor is "uname -m", not "uname -p" (#189958)

The comment in lib/systems/default.nix for uname.processor indicates that it
should match `uname -p`.  I tried that command and found that it reports
`unknown` on all of these machines:

- `x86_64-linux`
- `aarch64-linux`
- `mips64el-linux`
- `powerpc64le-linux`

The command `uname -m` reports the expected value on all of the above.

I think the comment is wrong.  So I fixed it.

lib/systems/default.nix

@@ -99,7 +99,7 @@ rec {
           genode = "Genode";
         }.${final.parsed.kernel.name} or null;
 
-         # uname -p
+         # uname -m
          processor = final.parsed.cpu.name;
 
          # uname -r